A used system carries the history of the car it came off. A car that was lowered, driven hard or repaired after an impact presents differently from one that was standard and gently used, and the difference shows in the parts that photograph badly: flanges, hangers and welds.
Ask what happened to the car and why the system came off. A system removed for an upgrade is a different proposition from one removed because the car was written off, and the secondhand exhaust pipework in each case deserves different scrutiny.
A flange tells you whether the joint was sealed and how many times it was disturbed. A flat, bright face with an even crush pattern says the joint was cared for. A face with deep gouges, a dished centre or sealant residue says somebody was chasing a leak.
Lay a straight edge across the flange and look for light underneath. Warping from heat or from being prised apart is common and never visible in a photograph. A flange that will not sit flat will not seal, however good the rest of the system is.
On stainless and titanium, the colour around a weld records how much oxygen reached the metal while it was molten. A clean straw or light blue tint is normal. Heavy grey or black oxidation means the weld was made without enough shielding, and that is where cracking starts.
Look at the inside of the weld too, if you can see it, since that is where exhaust gas and condensate work on the weakest surface. Cracks at the toe of a weld are the failure to look for, and they usually show as a fine dark line before anything separates.
A stainless steel exhaust muffler can look perfect and be half blocked or have a baffle loose inside. Shake it and listen for anything that moves; a rattle means an internal part has come adrift and will only get worse with heat.
Look for heat discolouration in a pattern that suggests a restriction, and for condensate staining at the lowest point, where internal corrosion begins. A muffler that has been run mostly on short trips is a worse prospect than one that has covered long distances.
Dents reduce section and change how the pipe fits. A small dent in a straight run is cosmetic; a dent on the inside of a bend is a restriction and a stress riser. Check the whole length, including the top surface that is hidden when the system is laid out on the ground.
Check that the pipe is still round at every joint, since ovalised ends leak. Look at the hangers as well: bent or stretched hangers mean the system was hanging badly or was pulled off in a hurry, and the mounting points on the car may be distorted to match.
Value the system against what it will take in time and parts to make it fit and seal. If the flanges are flat, the welds are clean and the muffler is sound, a used system can be a sensible buy. If two of those are doubtful, the money is better spent elsewhere.
Set your own limit before you go, and inspect in this order: flange, weld, muffler, shape, hangers. Anything that fails more than one check is a project rather than a part, and a project is only worth taking on if you wanted one.
